This invention relates to improved detectors especially for industrial and medical computerized tomography, and more particularly to a collimator system for X-ray detectors which provides much higher resolution than a stand alone X-ray detector.
Industrial X-ray Computerized Tomography (CT) systems are designed to provide images of the internal structure of manufactured parts. In order to effectively image small. structures or defects, high resolution measurements are required. A major limitation to the resolution achievable in the past is found in the structure of the X-ray detector itself. This invention comprises a way of dramatically improving the resolution of the detector system.
Typical X-ray detectors in current use for CT systems are Xenon ionization chambers. The operation of such detectors is described in U.S. Pat. No. 4,394,578 and other commonly assigned U.S. patents of J. M. Houston and N. R. Whetten. It consists essentially of a parallel plate capacitor with high pressure Xenon gas as the dielectric. A printed circuit board implementation consists of a high voltage plate at a fixed separation from a ground plate. The incident X-rays ionize the Xenon gas, and the ions are attracted to the voltage plates. The charge is collected and transferred to a charge to voltage converter, and the resulting voltage is digitized and sent to a computer. The amount of charge collected is proportional to the incident X-ray flux. If more than one detector is required, as is the case for CT systems, the ground plate contains several metallized regions, separated by narrow insulating strips.
The resolution of the detector is determined, to first order, by the width of the metallized regions. The Xenon ions are attracted by the electric field to the nearest plate of opposite charge. For wide metal regions, this is a sufficient approximation. For narrow metal strips, however, the spread of the primary electrons and other radiations produced by the X-rays in the detector must be taken into account. Because of the effect, the ions do not always arrive at the nearest metal strip. The ions can, in fact, find their way to strips several elements away. This effect, known as crosstalk, limits the resolution of these detectors rather severely as metal strip width becomes small. It has been shown that for strips on 0.003 inch centers, the response of a neighboring channel is about 65% of the response of the desired channel. This behavior is a serious problem if very high resolution is required.